June 16, 2009

Controllable DNA binding on Nanoparticles

The novel DNA 'sticky ends' can form intra-particle loops and hairpins (e.g. schemes II & III), giving more control over the particles' interactions than conventional sticky ends that can only form inter-particle bridges (scheme Ia).

Their work, which appears in the latest issue of the journal Nature Materials, overcomes the problem of uncontrollable sticking, which had been a barrier to the successful creation of stable microscopic and macroscopic structures with a sophisticated architecture.

In order to obtain self-replication, the researchers coat micrometer-sized particles with short stretches of DNA, so-called "sticky ends". Each sticky end consists of a particular sequence of DNA building blocks and sticky ends with complementary sequences form very specific bonds that are reversible. Below a certain temperature, the particles recognize each other and bind together, while they unbind again above that temperature. This enables a scheme in which the particles spontaneously organize into an exact copy on top of a template, which can then be released by elevating the temperature.

Scientists have used DNA-mediated interactions before, but it has always been very difficult to bind only a subset of particles—usually, either all particles or no particles are bound. This makes it challenging to make well-defined structures. Therefore, the NYU team, comprised of researchers in the Physics Department's Center for Soft Matter Research and in the university's Department of Chemistry, sought to find a method to better control the interactions and organization of the particles.

“We can finely tune and even switch off the attractions between particles, rendering them inert unless they are heated or held together—like a nano-contact glue,” said Mirjam Leunissen, a post-doctoral fellow in the Center for Soft Matter Research and the study’s lead author.

To maneuver the particles, the team used optical traps, or tweezers. This tool, created by David Grier, chair of NYU’s Department of Physics and one of the paper’s authors, uses laser beams to move objects as small as a few nanometers, or one-billionth of a meter.

The work has a range of possible applications. Notably, because the size of micrometer-scale particles—approximately one-tenth the thickness of a strand of human hair—is comparable to the wavelength of visible light, ordered arrays of these particles can be used for optical devices. These include sensors and photonic crystals that can switch light analogous to the way semi-conductors switch electrical currents. Moreover, the same organizational principles apply to smaller nanoparticles, which possess a wide range of electrical, optical, and magnetic properties that are useful for applications.

The researchers took advantage of the ability of certain DNA sequences to fold into a hairpin-like structure or to bind to neighboring sticky ends on the same particle. They found that if they lowered the temperature very rapidly, these sticky ends fold up on the particle—before they can bind to sticky ends on other particles. The particles stuck only when they were held together for several minutes—a sufficient period for the sticky ends to find a binding partner on another particle.

Surface functionalization with DNA is a powerful tool for guiding the self-assembly of nanometre- and micrometre-sized particles. Complementary 'sticky ends' form specific inter-particle links and reproducibly bind at low temperature and unbind at high temperature. Surprisingly, the ability of single-stranded DNA to form folded secondary structures has not been explored for controlling (nano) colloidal assembly processes, despite its frequent use in DNA nanotechnology. Here, we show how loop and hairpin formation in the DNA coatings of micrometre-sized particles gives us in situ control over the inter-particle binding strength and association kinetics. We can finely tune and even switch off the attractions between particles, rendering them inert unless they are heated or held together—like a nano-contact glue. The novel kinetic control offered by the switchable self-protected attractions is explained with a simple quantitative model that emphasizes the competition between intra- and inter-particle hybridization, and the practical utility is demonstrated by the assembly of designer clusters in concentrated suspensions. With self-protection, both the suspension and assembly product are stable, whereas conventional attractive colloids would quickly aggregate. This remarkable functionality makes our self-protected colloids a novel material that greatly extends the utility of DNA-functionalized systems, enabling more versatile, multi-stage assembly approaches.